Let my son go, that he may serve me - Which they could not do in Goshen, consistently with the policy and religious worship of the Egyptians; because the most essential part of an Israelite's worship consisted in sacrifice, and the animals which they offered to God were sacred among the Egyptians. Moses gives Pharaoh this reason Exodus 8:26.

I will slay thy son, even thy first-born - Which, on Pharaoh's utter refusal to let the people go, was accordingly done; see Exodus 12:29.

And I say unto thee, let my son go, that he may serve me,.... Worship God according to his will in the place he had designed for him, and where he might be safe and free; and which service was due from him as a son, and to be performed not in a servile way, but in a filial manner, and therefore as a servant he could demand his dismission, and much more as his son; and this is required in an authoritative way, for saying is here commanding, insisting on it as a point of right to be done:

and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn; meaning, not only in a strict and literal sense Pharaoh's firstborn son, and heir to his crown, but the firstborn of all his subjects, which in a civil sense were his. This was not to be said to Pharaoh at the first opening of his commission to him, but after all methods had been tried, and the several other plagues designed were inflicted on him to no purpose, he was to be told this, which was the last plague, and succeeded; but this is told to Moses before hand, that when other messages he should be sent with to him, and all that should be done by him would prove ineffectual, this, when sent with and performed, would have the desired effect.

And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.

4:23 Let my son go - Not only my servant whom thou hast no right to detain, but my son whose liberty and honour I am jealous for. If thou refuse, I will slay thy son, even thy first - born - As men deal with God's people, let them expect to be themselves dealt with.

4:18-23 After God had appeared in the bush, he often spake to Moses. Pharaoh had hardened his own heart against the groans and cries of the oppressed Israelites; and now God, in the way of righteous judgment, hardens his heart against the teaching of the miracles, and the terror of the plagues. But whether Pharaoh will hear, or whether he will forbear, Moses must tell him, Thus saith the Lord. He must demand a discharge for Israel, Let my son go; not only my servant, whom thou hast no right to detain, but my son. It is my son that serves me, and therefore must be spared, must be pleaded for. In case of refusal I will slay thy son, even thy first-born. As men deal with God's people, let them expect so to be dealt with.